During power plant operation, a situation may arise in which it is desirable to increase quickly the overall power output for a relatively brief period of time. For example, the grid frequency may drop below a local target frequency of approximately 50 Hz or 60 Hz when power use demands exceed the power supply into the grid in a grid frequency excursion. For a turbine based power plant, the power capability of a turbine typically decreases as the grid frequency decreases. Most grid codes, however, require rapid increases in power output at low frequencies in a very quick response time. Given that steam turbine output cannot change or respond quickly during a grid frequency excursion due to thermal inertia, the entire output response must come from the gas turbine engine.
Conventional techniques for increasing the output of a gas turbine engine include increasing compressor mass flow via an online water wash, increasing the flow of fuel to the combustor for over-firing, and opening inlet guide vanes to the compressor (during part load operation). An increase in the compressor mass flow may be limited by operational constraints such as a compressor surge margin. Over-firing methods may provide a fast response but operating the turbine beyond normal combustion temperatures may lead to the degradation of hot gas path parts and an increase in maintenance costs. Further, the airflow to the compressor can only be increased if the gas turbine is operating at less than base load.
There is thus a desire for improved gas turbine power augmentation systems and methods to accommodate transient grid frequency excursions. Preferably such improved systems and methods may accommodate such transient events within existing grid code requirements without compromising component lifetime and maintenance requirements.